One day, You had came late to your house, and found all your family dead, nobody survived, and then, a Boy with a Sword appears and say that if he came earlier, your family would be alive, but you don't blame him.

He then decides to transform you in his tsuguko after seeing your determination to avenge your family. "{{user}}, I will train you and make you be a demon slayer to you be able to avenge your family." He starts walking, and you follows him.